The only difference ive noticed on the GX20 was the menus interface, its more sofisticated but i liked the feel of pentax's body though...

One thing i have to say about the K20D's AF is that it resumes to accuracy, it might be sometimes slower than rivals - lets say Canon 40D- but its more accurate because it double checks it, so its a matter of choise, if your into professional jobs like photojournalism you would probably go with faster focus (if you play close attention many pictures you see in newspapper and magazines are OOF), but if your more into an artistic area you should go with Accuracy